/*!
 * by zhangxinxu(.com) 2012-12-27
 * you can visit http://www.zhangxinxu.com/wordpress/?p=3855 to get more infomation
 * under MIT license
*/
define(function(require, exports, module) {
	var funParabola = function(element, target, options) {
		var defaults = {
			speed: 166.67, // 每帧移动的像素大小，每帧（对于大部分显示屏）大约16~17毫秒
			curvature: 0.001,  // 实际指焦点到准线的距离，你可以抽象成曲率，这里模拟扔物体的抛物线，因此是开口向下的
			progress: function() {},
			complete: function() {}
		};
		
		var params = {}; options = options || {};
		
		for (var key in defaults) {
			params[key] = options[key] || defaults[key];
		}
		
		var exports = {
			mark: function() { return this; },
			position: function() { return this; },
			move: function() { return this; },
			init: function() { return this; }
		};

		var moveStyle = "margin", testDiv = document.createElement("div");
		if ("oninput" in testDiv) {
			["", "ms", "webkit"].forEach(function(prefix) {
				var transform = prefix + (prefix? "T": "t") + "ransform";
				if (transform in testDiv.style) {
					moveStyle = transform;
				}
			});		
		}
		
		// 根据两点坐标以及曲率确定运动曲线函数（也就是确定a, b的值）
		/* 公式： y = a*x*x + b*x + c;
		*/
		var a = params.curvature, b = 0, c = 0;
		
		// 是否执行运动的标志量
		var flagMove = true;
		
		if (element && target && element.nodeType == 1 && target.nodeType == 1) {
			var rectElement = {}, rectTarget = {};
			
			// 移动元素的中心点位置，目标元素的中心点位置
			var centerElement = {}, centerTarget = {};
			
			// 目标元素的坐标位置
			var coordElement = {}, coordTarget = {};
			
			// 标注当前元素的坐标
			exports.mark = function() {
				if (flagMove == false) return this;
				if (typeof coordElement.x == "undefined") this.position();
				element.setAttribute("data-center", [coordElement.x, coordElement.y].join());
				target.setAttribute("data-center", [coordTarget.x, coordTarget.y].join());
				return this;
			}
			
			exports.position = function() {
				if (flagMove == false) return this;
				
				var scrollLeft = document.documentElement.scrollLeft || document.body.scrollLeft,
					scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
				
				// 初始位置
				if (moveStyle == "margin") {
					element.style.marginLeft = element.style.marginTop = "0px";
				} else {
					element.style[moveStyle] = "translate(0, 0)";
				}
				
				// 四边缘的坐标
				rectElement = element.getBoundingClientRect();
				rectTarget = target.getBoundingClientRect();
				
				// 移动元素的中心点坐标
				centerElement = {
					x: rectElement.left + (rectElement.right - rectElement.left) / 2 + scrollLeft,
					y: rectElement.top + (rectElement.bottom - rectElement.top) / 2	+ scrollTop
				};
				
				// 目标元素的中心点位置
				centerTarget = {
					x: rectTarget.left + (rectTarget.right - rectTarget.left) / 2 + scrollLeft,
					y: rectTarget.top + (rectTarget.bottom - rectTarget.top) / 2 + scrollTop		
				};
				
				// 转换成相对坐标位置
				coordElement = {
					x: 0,
					y: 0	
				};
				coordTarget = {
					x: -1 * (centerElement.x - centerTarget.x),
					y:  -1 * (centerElement.y - centerTarget.y)	
				};
				
				/*
				 * 因为经过(0, 0), 因此c = 0
				 * 于是：
				 * y = a * x*x + b*x;
				 * y1 = a * x1*x1 + b*x1;
				 * y2 = a * x2*x2 + b*x2;
				 * 利用第二个坐标：
				 * b = (y2+ a*x2*x2) / x2
				*/
				// 于是
				b = (coordTarget.y - a * coordTarget.x * coordTarget.x) / coordTarget.x;	
				
				return this;
			};		
			
			// 按照这个曲线运动
			exports.move = function() {
				// 如果曲线运动还没有结束，不再执行新的运动
				if (flagMove == false) return this;
				
				var startx = 0, rate = coordTarget.x > 0? 1: -1;

				var step = function() {
					// 切线 y'=2ax+b
					var tangent = 2 * a * startx + b; // = y / x
					// y*y + x*x = speed
					// (tangent * x)^2 + x*x = speed
					// x = Math.sqr(speed / (tangent * tangent + 1));
					startx = startx + rate * Math.sqrt(params.speed / (tangent * tangent + 1));
					
					// 防止过界
					if ((rate == 1 && startx > coordTarget.x) || (rate == -1 && startx < coordTarget.x)) {
						startx = coordTarget.x;
					}
					var x = startx, y = a * x * x + b * x;
					
					// 标记当前位置，这里有测试使用的嫌疑，实际使用可以将这一行注释
					element.setAttribute("data-center", [Math.round(x), Math.round(y)].join());
					
					// x, y目前是坐标，需要转换成定位的像素值
					if (moveStyle == "margin") {
						element.style.marginLeft = x + "px";
						element.style.marginTop = y + "px";
					} else {
						element.style[moveStyle] = "translate("+ [x + "px", y + "px"].join() +")";
					}
					
					if (startx !== coordTarget.x) {
						params.progress(x, y);
						window.requestAnimationFrame(step);	
					} else {
						// 运动结束，回调执行
						params.complete();
						flagMove = true;	
					}
				};
				window.requestAnimationFrame(step);
				flagMove = false;
				
				return this;
			};
			
			// 初始化方法
			exports.init = function() {
				this.position().mark().move();
			};
		}
		
		return exports;
	};
	
	module.exports = funParabola;

	/*! requestAnimationFrame.js
	 * by zhangxinxu 2013-09-30
	*/
	(function() {
	    var lastTime = 0;
	    var vendors = ['webkit', 'moz'];
	    for(var x = 0; x < vendors.length && !window.requestAnimationFrame; ++x) {
	        window.requestAnimationFrame = window[vendors[x] + 'RequestAnimationFrame'];
	        window.cancelAnimationFrame = window[vendors[x] + 'CancelAnimationFrame'] ||    // name has changed in Webkit
	                                      window[vendors[x] + 'CancelRequestAnimationFrame'];
	    }

	    if (!window.requestAnimationFrame) {
	        window.requestAnimationFrame = function(callback, element) {
	            var currTime = new Date().getTime();
	            var timeToCall = Math.max(0, 16.7 - (currTime - lastTime));
	            var id = window.setTimeout(function() {
	                callback(currTime + timeToCall);
	            }, timeToCall);
	            lastTime = currTime + timeToCall;
	            return id;
	        };
	    }
	    if (!window.cancelAnimationFrame) {
	        window.cancelAnimationFrame = function(id) {
	            clearTimeout(id);
	        };
	    }
	}());
})

